Using Java Servlets with Database Connectivity

The persistent nature of Java servlets makes them ideal for database/web technology. Mr. McDonald takes a look at using servlets with PostgreSQL and JDBC.

The Guest Book Application

My first guest-book application was a Perl CGI script that
kept all the guest entries in a formatted text file that was
displayed or added to as the parameters dictated—simple and
effective. Unfortunately, I wanted to do more with that
information, including linking back to the Apache log files and
maybe even handing out cookies to monitor usage. The script, which
started small and lean, began to choke on the file processing
necessary. It was also a little distressing, since I had a very
functional RDBMS installed and ready to use. I studied the
alternatives (mod_perl) and
decided to go with Servlets. The first step was to design the
tables. The most important table holds each guest-book entry on a
row.

Each row in the guest-book table is uniquely identified by
the entryid column which has an implicit index.
Additionally, the handling of these keys is handled by a key table,
a convenience in a small application but almost essential in larger
applications.

The key table is able to keep track of any number of keys within
the range of the int type, which in my case is
[-231,231-1]. In order to get a new key, the current key must be
retrieved and then incremented. This must be atomic, i.e., the
operation must take place in a transaction. This is illustrated in
the getKey method. Ideally, a
stored procedure would handle all these details (the key-table
concept should not be visible to application developers in an ideal
world), but this level of detail with PostgreSQL involves C shared
libraries and Database APIs—something with which I did not want to
get involved.

An additional problem that needed solving was how to provide
servlets with property files in a consistent fashion. Property
files provide a convenient way of placing data that would otherwise
be hard-coded into the application. This is done by providing a
property on the command line to JServ called base.dir which points
to a world-writable directory (but is
chmod +t so that user's may not stomp on
other users files). This is specified in the httpd.conf file in the
ServletBinaryArgument tag. In this directory you
can store property files which may be loaded by servlets. I am sure
this can be done in a smarter fashion.

The main processing entry point is the service method. From
here, either the list of entries is displayed
(listEntries), a form is displayed
(showForm) or a new entry is made
(addEntry). The code for the Guest
Book application is not shown due to space considerations, but is
included in the archive file at
ftp.linuxjournal.com/pub/lj/listings/issue67/3243.tgz.

Listing 4 is the property file used to set up the various
parts of the servlet. I moved a number of the HTML header and
footer strings into this file along with the JDBC configuration
parameters.

Conclusion

The servlet/JDBC/PostgreSQL proved to be a powerful and fast
technology. Most of the problems I encountered were configuration
problems that required me to carefully read the associated
documentation. Unfortunately, technical documentation on Servlets
is scarce and I would urge further experimentation. The next step I
took with this application was to install Sybase ASE for Linux, a
RDBMS with which I am comfortable. I then coded a number of stored
procedures that allowed the Servlet to delegate most of its data
manipulation to the database, where it rightly belongs. If you are
going to replace PostgreSQL with Sybase, it is necessary to get the
jConnect JDBC classes from the Sybase web site. Of course, this can
be done with PostgreSQL, but learning the details of an RDBMS C API
was tangential to the exercise.

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